Semiconductor quantum dot device

ABSTRACT

A p-type semiconductor barrier layer is provided in the vicinity of undoped quantum dots, and holes in the p-type semiconductor barrier layer are injected in advance in the ground level of the valence band of the quantum dots. Lowering the threshold electron density of conduction electrons in the ground level of the conduction band of quantum dots in this way accelerates the relaxation process of electrons from an excited level to the ground level in the conduction band.

TECHNICAL FIELD

The present invention relates to electronic devices such as field effecttransistors and single-electron elements as well as to semiconductoroptical devices such as semiconductor lasers, semiconductor amplifiers,semiconductor optical switches, wavelength conversion elements, andoptical memories which are used in, for example, optical communicationand optical interconnections; and more particularly to a semiconductorquantum dot device which is provided with quantum dots.

BACKGROUND ART

Quantum dots can be called boxes of quantized potential on the hyperfinescale in which carriers (conduction electrons and holes) are confined inthree dimensions. The density of the state function of carriers in aquantum dot is broken up according to a delta function, and the injectedcarriers are concentrated in these discrete energy levels. As a result,light which is emitted when the conduction electrons and holes recombinein the interior of quantum dots is of high optical intensity, andmoreover, the width of the spectrum is extremely narrow.

In a semiconductor laser in which quantum dots having these propertiesare provided in an active layer, the reduced threshold current which isrequired for laser oscillation and the decrease in carriers results inreduced internal loss, and therefore, improved oscillation efficiency.In addition, the marked change in gain peak with respect to change inthe carrier density enables high-speed operation. (A semiconductor laserin which quantum dots are applied in the active layer is hereinbelowreferred to as a “quantum dot laser.”)

The realization of a quantum dot laser having these superior propertiesrequires that quantum dots be formed at high density and with highuniformity, and moreover, without degrading crystallinity. Methods thatare known from the prior art for forming such quantum dots includemethods which employ such techniques as lithography and dry etching.However, these methods have the drawback that processing damage whichoccurs in the semiconductor crystal as a result of lithography or dryetching greatly reduces the light emission efficiency.

As a method of eliminating this problem, a “Direct formation method (orSelf-organizing method)” has been developed and put into actual use inrecent years. In this method, quantum dots are formed merely by growingcrystals.

The direct formation method was reported by D. Leonard et al. in 1993.This method was proposed based on the finding that when a lattice-misfitInGaAs is grown on a GaAs substrate, the InGaAs grows as a layer untilit exceeds a critical film thickness, and upon exceeding the criticalfilm thickness, grows as islands, these island InGaAs crystals having asize of several tens of nanometers. This size is suitable for quantumdots [D. Leonard et al., Applied Physics Letters, Vol. 63, No. 23, pp.3203–3205, December 1993]. The direct formation method was subsequentlyconfirmed to be an excellent method for forming quantum dots.

The application of quantum dots formed by the direct formation method tosemiconductor lasers is currently being widely studied. For example, G.T. Liu et al. have demonstrated that a quantum dot laser fabricatedusing the direct formation method operates with a lower injected currentdensity than a prior-art semiconductor laser having an active layer of abulk structure or quantum well structure [G. T. Liu et al., ElectronicsLetters, Vol. 35, No. 14, pp. 1163–1165, Jul. 8, 1999].

Further, Chen has demonstrated that the operating current of a quantumdot laser which has been fabricated using the direct formation method isnot influenced by temperature [H. Chen et al. Electronics Letters, Vol.36, No. 20, pp. 1703–1704, Sep. 28, 2000].

However, a quantum dot laser which has an operating speed surpassingthat of a semiconductor laser of the prior art having an active layer ofbulk structure or quantum well structure has not yet been proposed. Thisis due to problems described hereinbelow that are inherent to quantumdots.

According to the Pauli exclusion principal, only two conductionelectrons and two holes can exist in each of the lowest energy levels(ground levels) of the conduction band and valence band in a quantum dotwhich has been quantized in three dimensions. As a result, if the groundlevels of the conduction band and valence band are already occupied byconduction electrons and holes (carriers), additional conductionelectrons or holes cannot be injected in the ground level. Thetransition of conduction electrons and holes from excited levels to theground levels of the conduction band and valence band is referred to as“relaxation.”

Accordingly, in a quantum dot laser which is oscillated by the injectionof a multiplicity of carriers into a plurality of quantum dots formed inan active layer, the relaxation rate of electrons which transition fromexcited levels of the conduction band (an energy level which is higherthan the ground level of the conduction band) to the ground levelsteadily drops with increase in the number of injected conductionelectrons. On the other hand, the relaxation rate of holes whichtransition from an excited level of the valence band (an energy levelwhich is lower than the ground level of the valence band) to the groundlevel drops steadily with increase in the number of holes that areinjected.

FIG. 1 is a graph showing the emission (photoluminescence, PL) spectrumof a semiconductor sample in which a plurality of InAs quantum dots areformed on a GaAs substrate. The emission spectrum shown in FIG. 1 showsthe results of measuring light which is emitted when excitation light isirradiated onto the sample to cause the generation of carriers insidethe sample and the recombination of these generated carriers inside theInAs quantum dots. In addition, FIG. 2 is a graph showing the relationbetween the excitation power and the ratio (I₂/I₁) of emission intensityI₁ from the ground level to emission intensity I₂ from the excited levelof the quantum dots for the sample that produced the emission spectrumof FIG. 1.

As can be understood from FIG. 1 and FIG. 2, as the excitation powerincreases, emission intensity I₁ from the ground level first increasesand gradually approaches saturation, following which emission intensityI₂ from the excited level increases. The excited level emissionintensity I₂ begins to gradually increase before the emission intensityI₁ from the ground level reaches complete saturation.

Here, if the thermal excitation of carriers to excited levels isconsidered, the change over time in the number of carriers at an excitedlevel can be given by the following equation (1):

$\begin{matrix}{\frac{\partial N_{1}}{\partial t} = {\frac{N_{2}}{\tau_{r2}} - \frac{N_{1}}{\tau_{r2}} - \frac{N_{1}}{\frac{\tau_{2 - 1}}{\exp\left( {- \frac{\Delta\; E_{2 - 1}}{kT}} \right)}}}} & (1)\end{matrix}$

In equation (1), N₁ and N₂ represent the number of carriers at theground level and excited level, respectively; τ_(r1) and τ_(r2)represent the radiative recombination times of the ground level andexcited level, respectively; τ₂₋₁ is the relaxation time of carriersfrom the excited level to the ground level; ΔE₂₋₁ represents the energydifference between the excited level and the ground level; k is theBoltzmann constant; and T is the absolute temperature.

In a stationary state, the left side of equation (1) is “0” and theratio (I₂/I₁) of excited level emission intensity I₂ to ground levelemission intensity I₁ can therefore be shown by equation (2) below:

$\begin{matrix}{\frac{I_{2}}{I_{1}} = {\frac{\frac{N_{2}}{\tau_{r2}}}{\frac{N_{1}}{\tau_{r1}}} = \frac{\tau_{2 - 1} + {\tau_{r1}\mspace{11mu}{\exp\left( {- \frac{\Delta\; E_{2 - 1}}{kT}} \right)}}}{\tau_{r2}}}} & (2)\end{matrix}$

As can be understood from equation (2), emission intensity ratio (I₂/I₁)is a function of the absolute temperature T.

In the above-described semiconductor sample (in which InAs quantum dotsare formed on a GaAs substrate), if ΔE₂₋₁ is assumed to be 73 meV,τ_(r1) to be 0.7 ns and τ_(r2) to be 0.5 ns at room temperature (293 K),the relation of the excitation power to relaxation time τ₂₋₁ ofcarriers, which is found from the change in the emission intensity ratio(I₂/I₁) shown in FIG. 2 is as shown in FIG. 3.

As can be understood from FIG. 3, as the excitation power increases, therelaxation time τ₂₋₁ of the carriers gradually increases due to fillingof carriers at the ground level and thus approaches the radiativerecombination times τ_(r1) and τ_(r2) (approximately 1 ns) of the groundlevel and excited level.

Thus, when the excitation power is 100 W/cm², i.e., when the samecarrier density can be obtained as during laser oscillation, the carrierrelaxation time τ₂₋₁ becomes 0.2 ns at room temperature (293 K). In thiscase, the 3-dB modulation bandwidth f_(3dB) given by the followingequation (3) is 0.8 GHz. In other words, the 3-dB modulation bandf_(3dB), which is the frequency range in which direct modulation ispossible, is limited to 0.8 GHz.

$\begin{matrix}{f_{3{dB}} = \frac{1}{2\;\pi\;\tau_{2 - 1}}} & (3)\end{matrix}$

As is obvious from the foregoing explanation, the modulation rate in aquantum dot device such as a quantum dot laser drops due to decrease inthe relaxation rate of carriers (increase in carrier relaxation timeτ₂₋₁), and this limited modulation rate complicates the realization of amodulation bandwidth on the order of 10 GHz, which is required in acurrent optical communication system.

The present invention was developed to overcome the above-describedproblems inherent to the prior art and has as an object the provision ofa quantum dot device which, by accelerating the relaxation of carriersto the ground level in quantum dots, can realize a broad modulation bandof at least approximately 10 GHz and high-speed operation.

DISCLOSURE OF THE INVENTION

The quantum dot device of the present invention is a construction inwhich a p-type impurity is injected into quantum dots, whereby thisp-type impurity causes the generation of holes inside the quantum dots,these holes being at the ground level of the valence band of the quantumdots. In such a construction, the relaxation of carriers (carrierinjection) from an excited level to the ground level is accelerated, andincrease in the relaxation time of the carriers is therefore suppressed.As a result, the modulation band of the quantum dot device can beextended and a broad modulation band on the order of 10 GHz or more andan operating speed on the order of 10 GHz or higher can be realized.

In the present invention, the following relation is satisfied in theabove-described quantum dot device:X₂≧X₁,where X₁ (cm⁻²) is the surface density of the quantum dots and X₂ (cm⁻²)is the surface density of the p-type impurity contained in the quantumdots.

As a result, relaxation process of electrons from the excited level tothe ground level is accelerated in the conduction band of the quantumdot.

Furthermore, if the following relation is satisfied:X ₂24 2×X ₁,the relaxation of carriers (carrier injection) to the ground level isremarkably accelerated since substantially all the ground levels of thevalence band of the quantum dots are occupied by holes.

In addition, when the absorption loss by the hole excessively doped tothe ground level of the valence band is considered, it is preferable tosatisfy the following relation:X ₂≈2×X ₁.

Further, the quantum dot device of the present invention is aconstruction in which a p-type impurity region is formed in the vicinityof a quantum dots, whereby holes generated in the p-type impurity regionare introduced into the quantum dots, these holes being at the groundlevel of the valence band of the quantum dots. Here, the “vicinity” ofthe quantum dot includes cases in which the p-type impurity region isformed in contact with the quantum dot and cases in which the p-typeimpurity region is formed separated by a distance within which theinjection efficiency of the holes does not drop excessively, such as adistance of 10 nm or less.

In an exemplary embodiment, a semiconductor quantum dot device isprovided with a plurality of quantum dots and includes a p-type impurityregion disposed adjacent to the quantum dots, a p-type impurity beinginjected into the p-type impurity region for generating holes forfilling a ground level of a valence band of the quantum dots, whereinthe p-type impurity region is formed on a p-type semiconductor wettinglayer which is adjacent to the quantum dots.

Further, the p-type impurity region may be formed in contact with endsof the quantum dots and at a position which confronts a p-typesemiconductor wetting layer which is adjacent to the quantum dots withthe quantum dots interposed.

Additionally, a semiconductor buried layer is provided which buries gapsbetween the quantum dots. A band gap of the p-type impurity region issmaller than a band gap of the semiconductor buried layer.

In such a construction as well, the relaxation of the carriers (carrierinjection) from an excited level to the ground level is accelerated, andincreases in the relaxation time of the carriers can therefore besuppressed. As a result, the modulation band of the quantum dot devicecan be extended and a broad modulation band on the order of 10 GHz ormore and an operating speed on the order of 10 GHz or higher can berealized.

In the present invention, the following relation is satisfied in theabove-described quantum dot device:X₂≧X₁,where X₁ is the surface density of the quantum dots and X₂ is thesurface density of the p-type impurity contained in the p-type impurityregion.

As a result, relaxation process of electrons from the excited level tothe ground level is accelerated in the conduction band of the quantumdot.

Furthermore, if the following relation is satisfied:X ₂≧2×X ₁,the relaxation of carriers (carrier injection) to the ground level isremarkably accelerated since substantially all the ground levels of thevalence band of the quantum dots are occupied by holes.

In addition, when the absorption loss by the hole excessively doped tothe ground level of the valence band is considered, it is preferable tosatisfy the following relation:X ₂≈2×X ₁.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the emission photoluminescence spectrum of asemiconductor sample in which a plurality of InAs quantum dots have beenformed on a GaAs substrate;

FIG. 2 is a graph showing the relation between the excitation power andratio (I₂/I₁) of emission intensity I₂ from the excited level andemission intensity I₁ from the ground level of the quantum dots for thesample in which the emission spectrum of FIG. 1 is obtained;

FIG. 3 is a graph showing the relation between the excitation power andthe relaxation time τ₂₋₁ of carriers from an excited level to the groundlevel for the sample in which the emission spectrum of FIG. 1 isobtained;

FIG. 4A is a sectional view showing the construction of the firstembodiment of the semiconductor quantum dot device of the presentinvention;

FIG. 4B is an energy band diagram of the semiconductor quantum dotdevice shown in FIG. 4A;

FIG. 5 is a graph showing the relation between the injected current and3-dB modulation band f_(3dB) in a quantum dot laser having an ordinaryconstruction in which holes are not injected in advance into quantumdots;

FIG. 6 is a graph showing the relation between the injected current and3-dB modulation band f_(3dB) in a quantum dot laser having theconstruction of the present invention in which holes are injected inadvance into quantum dots;

FIG. 7A is a sectional view showing the construction of the secondembodiment of the semiconductor quantum dot device of the presentinvention;

FIG. 7B is an energy band diagram of the semiconductor quantum dotdevice shown in FIG. 7A;

FIG. 8 is a sectional view showing the construction of the first exampleof a quantum dot laser in which the semiconductor quantum dot device ofthe present invention has been applied.

FIG. 9 is a sectional view showing the construction of the secondexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied;

FIG. 10 is a sectional view showing the construction of the thirdexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied;

FIG. 11 is a sectional view showing the construction of the fourthexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied; and

FIG. 12 is a sectional view showing the construction of the fifthexample of a quantum dot laser in which the semiconductor quantum dotdevice of the present invention has been applied.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is next described with reference to theaccompanying drawings.

[First Embodiment]

As shown in FIG. 4A, the semiconductor quantum dot device according tothe first embodiment is a construction in which a plurality of quantumdots 1 which have been formed from an undoped semiconductor are disposedon the surface of p-type semiconductor barrier layer 3 at a surfacedensity of, for example, 5×10 cm⁻².

P-type impurity is injected into p-type semiconductor barrier layer 3,whereby a multiplicity of holes 4 exist in the interior of p-typesemiconductor barrier layer 3. A portion of these holes 4 are introducedinto adjacent quantum dots 1. In other words, this is a construction inwhich quantum dots 1 shown in FIG. 4A contain holes 4 even though thesequantum dots 1 are not injected with p-type impurity (i.e., the quantumdots are undoped). As shown in FIG. 4B, these holes 4 are in the groundlevel of the valence band of quantum dots 1, and the ground level of thevalence band is therefore filled by holes 4. In addition, quantum dots 1are covered by undoped semiconductor barrier layer 2 formed over quantumdots 1.

As described in the foregoing explanation, the ground level of thevalence band of quantum dots 1 includes a number of states of two if thespin is taken into consideration. The amount of p-type impurity injectedinto p-type semiconductor barrier layer 3 is therefore set to generateholes at a surface density of 1×10¹¹ cm⁻² (=5×10¹⁰ cm⁻²×2).

In the present embodiment, holes 4 are introduced into quantum dots 1 inadvance and the presence of these holes 4 in the ground levels of thevalence band of quantum dots 1 accelerates the process of relaxation ofconduction electrons 5 from excited levels to the ground levels of theconduction band of quantum dots 1. In other words, the relaxation timeof conduction electrons 5 of quantum dots 1 is reduced.

It should be noted that the “radiative recombination of holes andconduction electrons” shown in FIG. 4B means the process in whichconduction electrons 5 transition from the ground level of theconduction band to the ground level of the valence band of quantum dots1, recombine with holes 4 which are at the ground level of the valenceband, and thus emit light.

The reason for the acceleration of the relaxation process of conductionelectrons 5 from an excited level to the ground level of the conductionband of quantum dots 1 is explained hereinbelow. The followingexplanation regards a case in which the quantum dot device of thepresent embodiment is applied to a semiconductor laser, but theexplanation also holds true when the quantum dot device of the presentinvention is applied to other optical devices.

Typically, when directly modulating a semiconductor laser, the injectedcarriers, i.e., the injected current, are increased or decreased inaccordance with modulation signals while the semiconductor laser issubjected to laser oscillation. Since the threshold value of the carrierdensity of the active layer (the threshold carrier density) is constantduring laser oscillation, the intensity of emitted light varies inaccordance with the increase and decrease of the carrier.

In the case of a quantum dot laser which includes a quantum dotstructure in the active layer, however, the threshold carrier density ofthe ground level of the quantum dots should be reduced to a minimum andthe relaxation process of the carrier to the ground level should beaccelerated in order to improve the operating (modulation) speed.

As shown in FIG. 4B, in quantum dot 1, conduction electrons 5 in theconduction band are injected in the ground level of the conduction band,and holes 4 of the valence band are injected in the ground level of thevalence band. Since the effective mass of hole 4 is greater by onedecimal place than the effective mass of conduction electron 5, theenergy difference between the ground level and the excited level of thevalence band is extremely small and holes 4 are easier to relax thanconduction electrons 5. In other words, holes 4, which have greatereffective mass, have properties which are easier to restrain in aquantum level such as the ground level or excited level than electrons,which have less effective mass. It can be seen that the difficulty ofrelaxing conduction electron 5, which have smaller effective mass, isthe governing factor which limits the high-speed modulation of thesemiconductor laser.

Therefore, the time necessary for the relaxation process of conductionelectrons 5 from an excited level to the ground level of the conductionband (i.e., the relaxation time) should be shortened as much as possibleto the. realize high-speed modulation characteristics of a quantum dotlaser.

However, when the density of states (number of states) of conductionelectrons 5 or holes 4 in quantum dots 1 is approximated using aGaussian function having an uneven spread ΔE from center energy E₀, gaing (the gain of only the ground level) of an active layer having aquantum dot structure is as shown in equation (4):

$\begin{matrix}{g = {\frac{h^{3}c^{2}N}{2\;\pi\; n^{2}E_{0}^{2}\tau_{sp}\Delta\; E}{\sqrt{\frac{\ln\; 2}{\pi}}\left\lbrack {f_{c} + \left( {1 - f_{v}} \right) - 1} \right\rbrack}}} & (4)\end{matrix}$

In equation (4), h is the Planck's constant, c is the speed of light, Nis the surface density of quantum dots 1, n is the index of refraction,τ_(sp) is the lifetime of spontaneous emission light, f_(c) is theoccupation probability of conduction electrons 5 in the conduction band,and f_(v) is the occupation probability of conduction electrons in thevalence band. In addition, f_(c) and f_(v) are values such that0≦f_(c)≦1, 0≦f_(v)≦1.

When the gain g expressed by equation (4) exceeds the loss in aresonator, laser oscillation occurs in a quantum dot laser. When theloss of the resonator is maintained at a constant value by adopting thesame structure, causing the probability f_(v) that conduction electrons5 occupy the ground level of the valence band to approach “0”, that is,causing the probability that electrons are not present in the groundlevel of the valence band (the probability that holes 4 occupy theground level of the valence band) to approach “1” is effective forlowering the probability f_(c) that conduction electrons 5 occupy theground level of the conduction band during laser oscillation.

Thus, to realize this result, holes 4 can be introduced into quantumdots 1 such that the ground level is occupied in advance by holes 4,whereby the probability f_(v) that conduction electrons 5 occupy theground level of the valence band of quantum dots 1 is decreased.

Further, in order to set the probability f_(v) that conduction electronsoccupy the ground level of the valence band to “0”, the amount of holes4 which are introduced into quantum dots 1 should be made equal to thenumber of states (density of states) of the ground level in quantum dots1.

In a semiconductor laser of the prior art which uses a quantum wellstructure or bulk structure as the active layer, however, it is possibleto lower the electron density during laser oscillation by appropriatelysetting the structure to inject holes into the active layer.

In a construction in which the conduction band extends continuously fromthe ground level toward higher energy, as in a semiconductor laser ofthe prior art, lowering the threshold electron density narrows thespread of energy which is held by the conduction electrons and narrowsthe spread of the gain energy, whereby the increase in the gain peak dueto increase in the electrons which are injected becomes greater and themodulation speed of the laser output becomes higher. These points havealready been reported by Uomi et al. [K. Uomi et al., Japanese Journalof Applied Physics, Vol. 29, No. 1, pp. 81–87, January 1990].

In the semiconductor laser provided with the quantum dots of the presentembodiment, however, high-speed modulation is achieved by principleswhich are entirely different from those in the semiconductor laser whichuses the above-described bulk structure as the active layer.Specifically, the quantization of quantum dots 1 in allthree-dimensional directions results in discrete energy levels and anextremely narrow energy spread. As a result, the spread of the gainenergy is uniform regardless of the amount of injected carriers, and thepeak value of gain g of an active layer having a quantum dot structurevaries linearly with respect to the electron density.

According to the above-described concepts relating to a semiconductorlaser which uses a quantum well structure or bulk structure as theactive layer, the modulation speed should be uniform and should notdepend on the electron density. In the semiconductor laser provided withthe quantum dots of the present embodiment, however, the ground level ofthe conduction band is filled by conduction electron 5, and the time(relaxation time) required for the relaxation process of conductionelectrons 5 which are in an excited level to the ground level istherefore greater.

Research by the present inventor confirmed that this lengthening of therelaxation time of conduction electrons 5 causes a drop in themodulation speed of a quantum dot laser. The introduction of holes 4 inadvance into the ground level of the valence band in quantum dots 1 andthe consequent reduction of the electron density of the ground level ofthe valence band of quantum dots 1 is effective for circumventing thisproblem.

FIG. 5 shows the relation between the injected current and 3-dBmodulation band f_(3dB) in a quantum dot laser having an ordinaryconstruction in which holes 4 are not introduced into quantum dots 1.This graph plots 3-dB modulation band f_(3dB) (a frequency range inwhich the response drops only 3 dB) for a case of fabricating a quantumdot laser having an ordinary construction in which holes 4 are notintroduced into quantum dots 1 and then directly modulating this quantumdot laser by a small signal and measuring the frequency responsecharacteristics. It should be noted that I_(th) is the oscillationthreshold current of this quantum dot laser.

As can be understood from FIG. 5, in a quantum dot laser in which holes4 are not introduced into quantum dots 1, although 3-dB modulation bandf_(3dB) rises with increase in bias current I_(bias), 3-dB modulationband f_(3dB) has an upper limit of approximately 2.5 GHz.

In contrast, FIG. 6 shows the relation between injected current and 3-dBmodulation band f_(3dB) in a semiconductor laser provided with thequantum dots of the present embodiment in which holes 4 are introducedto quantum dots in advance. The graph of FIG. 6 was obtained by the sameprocedure as FIG. 5 with the exception of the introduction of holes 4 inquantum dots 1 in advance. In this quantum dot laser, the amount ofdoping of the p-type impurity in p-type semiconductor barrier layer(p-type impurity region) 3 was set to ten times the number of states ofthe ground level of the valence band of quantum dots 1. The ground levelof this valence band is therefore completely filled by holes 4. Inaddition, the semiconductor laser having the characteristics of FIG. 6is a construction having the same structure as the quantum dot laser ofordinary structure which was fabricated for measuring thecharacteristics of FIG. 5, with the exception of the provision of p-typesemiconductor barrier layer (p-type impurity region) 3.

As can be clearly understood from FIG. 6, in the quantum dot laseraccording to the present embodiment in which holes 4 have been injectedin advance into quantum dots 1, the upper limit of 3-dB modulation bandf_(3dB) is 5 GHz, which is twice the level (2.5 GHz) of the quantum dotlaser of ordinary structure, and the frequency band is extended to twicethat of the quantum dot laser of ordinary structure.

However, in the quantum dot laser which was fabricated for obtaining theresults of FIG. 6, the upper limit of 3-dB modulation band f_(3dB) doesnot reach 10 GHz. This problem is thought to be a consequence of settingthe amount of doping of the p-type impurity in p-type semiconductorbarrier layer (p-type impurity region) 3 to ten times the number ofstates of the ground level of the valence band of quantum dots 1, thatis, doping which exceeds the amount of p-type impurity which is assumedto be necessary for filling the ground level of the valence band withholes 4. Essentially, this problem arises because excessive doping ofholes 4 in the ground level of the valence band of quantum dots 1 leadsto absorption loss, and thus leads to a slight increase in theoscillation threshold carrier density.

The amount of doping of p-type impurity in p-type semiconductor barrierlayer (p-type impurity region) 3 is therefore preferably regulated suchthat the amount of doping equals the number of states of the groundlevel of the valence band of quantum dots 1, that is, such that holes 4fill all of the number of states which can be obtained in the groundlevel of the valence band. In this way, a broad modulation band on theorder of 10 GHz becomes possible and high-speed operation on the orderof 10 GHz or greater can be realized.

As described in the foregoing explanation, decreasing the density ofconduction electrons 5 of the ground level of quantum dots 1 in aquantum dot laser having the construction of the present embodimentenables an acceleration of the relaxation process of conductionelectrons 5 from an excited level to the ground level of the conductionband of quantum dots 1. For example, if the probability f_(c) thatconduction electrons 5 occupy the ground level of the conduction bandcan be lowered to approximately 0.1, the relaxation time τ₂₋₁ ofconduction electrons 5 can be made approximately 10 ps. According to theabove-described equation (2), it can be seen that 3-dB modulation bandf_(3dB) at this time surpasses 10 GHz.

[Second Embodiment]

FIG. 7A is a view showing the construction of the second embodiment ofthe semiconductor quantum dot device according to the present invention,and FIG. 7B is an energy band diagram of this quantum dot device.

The quantum dot device of the second embodiment is a construction inwhich quantum dots 1 are formed on undoped semiconductor barrier layer 3a, and p-type semiconductor barrier layer (p-type impurity region) 2 ais formed on quantum dots 1. Holes 4 which are generated by the p-typeimpurity introduced into p-type semiconductor barrier layer 2 a areintroduced into quantum dots 1. In the quantum dot device according tothis second embodiment as well, action and effects can be obtained thatare equivalent to the action and effects of the first embodiment.

In addition, although examples were presented in above-described thefirst and second embodiments in which a p-type impurity is doped in asemiconductor barrier layer (p-type impurity region) which is disposedin the vicinity of quantum dots 1 and holes are introduced into thequantum dots from this semiconductor barrier layer, the holes may alsobe introduced into the quantum dots by directly doping the p-typeimpurity inside the quantum dots. According to experimentation by thepresent inventor, however, it was found that this type of compositionsuffers from the occurrence of flaws in the quantum dots and a drop inthe emission efficiency.

EXAMPLES

Explanation next regards examples of the present invention withreference to the accompanying drawing.

First Example

FIG. 8 shows the construction of a first example of a quantum dot laserin which the semiconductor quantum dot device of the present inventionhas been applied.

The quantum dot laser shown in FIG. 8 was fabricated by the followingprocedure which used an MBE (Molecular Beam Epitaxy) apparatus.

First, n-type AlGaAs cladding layer 22 (aluminum composition of 0.3,thickness of 3 μm, and carrier concentration of 1×10¹⁸ cm⁻³), undopedGaAs optical confinement layer 23 (thickness of 0.15 μm), and undopedGaAs barrier layer 24 (thickness of 20 nm) are successively grown onn-type GaAs substrate 21.

The temperature of GaAs substrate 21 is next set to 490° C. and InAs isgrown on undoped GaAs barrier layer 24 while regulating the amount suchthat the InAs having a thickness equivalent to a three-atom layer isgrown. At this time, undoped InAs quantum well layer (referred to as a“wefting layer”) 25 is first formed to a thickness equivalent to one- ortwo-atom layer, following which InAs forms islands which exceed thecritical film thickness of distortion while beryllium (Be), which is thep-type impurity, is simultaneously supplied. In this way, a plurality ofp-type InAs quantum dots 26 which contain p-type impurity were formed onundoped InAs wefting layer 25.

The p-type InAs quantum dots which were thus obtained had a planar discshape and a surface density of 5×10¹⁰ cm⁻². In addition, each quantumdot had a diameter of 30 nm and a thickness (height) of 8 nm.

In the first example, beryllium, which is the p-type impurity, issupplied together with the InAs when growing the InAs quantum dots asdescribed above, and the beryllium is doped inside the quantum dotswhile regulating the amount of this supply such that the surface densityof the beryllium (1×10¹¹ cm⁻²) at this time is twice the surface densityof quantum dots 1 (5×10¹⁰ cm⁻²). Holes are thus introduced as carriersinto each quantum dot, and these holes are at the ground level of thevalence band of each quantum dot.

Next, using the same MBE apparatus, undoped GaAs barrier layer 27(thickness of 20 nm), undoped GaAs optical confinement layer 28(thickness of 0.15 μm), p-type AlGaAs cladding layer 29 (aluminumcomposition of 0.3, thickness of 2 μm, and carrier concentration of5×10¹⁷ cm⁻³), and p-type AlGaAs cap layer 30 (aluminum composition of0.15, thickness of 0.5 μm, and carrier concentration of 5×10¹⁸ cm⁻³)were successively grown on the quantum dot structure to obtain thequantum dot laser shown in FIG. 8.

As shown in FIG. 8, p-type InAs quantum dots 26 in the first example arecovered by undoped GaAs barrier layer 27. In addition, quantum dotstructure 70 is formed by undoped InAs quantum well layer (wettinglayer) 25 and a plurality of p-type InAs quantum dots 26 formed overthis undoped InAs quantum well layer 25. Further, quantum dot structure70 and undoped GaAs barrier layers 24 and 27 which have been formedabove and below this quantum dot structure 70 constitute active layer 71of the quantum dot laser of the present example.

As previously described, in the quantum dot laser of the first example,beryllium (Be), which is a p-type impurity, is doped in the quantum dotsto generate holes, and the ground level of the valence band of eachquantum dot is filled by these holes.

The ground level of the valence band of each quantum dot is thereforenot occupied by conduction electrons during laser oscillation, wherebythe density of conduction electrons in the quantum dots drops and therelaxation process of conduction electrons from an excited level to theground level of the conduction band of the quantum dots is accelerated.

In the first example, the relaxation rate of electrons was acceleratedto approximately 10 ps. This means that the semiconductor laser of thefirst example is capable of high-speed modulation of 10 GHz or more.

Although GaAs substrate 21 was used in the present example, an InPsubstrate may be used instead. In such a case, cladding layers 22 and29, optical confinement layers 23 and 28, and barrier layers 24 and 27may be formed of InAlGaAs, or may be formed of InGaAsP. In this type ofquantum dot laser, the emission wavelength from p-type InAs quantum dots26 exceeds 1.3 μm, and this quantum dot laser is therefore well suitedfor use as a light source for use in long-wavelength opticalcommunication.

Second Example

FIG. 9 shows the construction of the second example of a quantum dotlaser in which the semiconductor quantum dot device of the presentinvention is applied.

The quantum dot laser of the second example is a construction in whichundoped InAs quantum dots 36, p-type GaAs barrier layer 37, and p-typeGaAs optical confinement layer 38 are formed in place of p-type InAsquantum dots 26, undoped GaAs barrier layer 27, and undoped GaAs opticalconfinement layer 28, respectively, in the first example. Theconstruction is otherwise identical to that of the semiconductor laserof the first example, and constituent elements which are identical toelements in the first example are therefore identified by the samereference numbers and redundant detailed explanation is here omitted.

The quantum dot laser of the second example was fabricated by thefollowing process using an MBE apparatus as in the first example.

As in the first example, n-type AlGaAs cladding layer 22, undoped GaAsoptical confinement layer 23, undoped GaAs barrier layer 24, and undopedInAs wetting layer 25 are successively grown on n-type GaAs substrate21.

A plurality of undoped InAs quantum dots 36 (diameter of 30 nm,thickness of 8 nm, and surface density of 5×10¹⁰ cm⁻²) are next formedon undoped InAs wetting layer 25. In the present example, in contrastwith the first example, beryllium (Be) is not supplied during growth ofInAs quantum dots 36.

In the present example, p-type GaAs barrier layer 37 (thickness of 20nm) and p-type GaAs optical confinement layer 38 (thickness of 0.15 μm)are successively grown on undoped InAs quantum dots 36.

Finally, as in the first example, p-type AlGaAs cladding layer 29 andp-type AlGaAs cap layer 30 were grown on p-type GaAs optical confinementlayer 38 to obtain the quantum dot laser shown in FIG. 9.

As shown in FIG. 9, undoped InAs quantum dots 36 of the second exampleare covered by p-type GaAs barrier layer 37. In addition, quantum dotstructure 70 a is formed by undoped InAs quantum well layer (wettinglayer) 25 and the plurality of undoped InAs quantum dots 36 formed onthis undoped InAs quantum well layer 25. Furthermore, quantum dotstructure 70 a and undoped GaAs barrier layers 24 and 27 formed aboveand below this quantum dot structure 70 a constitute active layer 71 aof the quantum dot laser of the present example.

Beryllium, which is the p-type impurity, is doped to a concentration of6×10¹⁵ cm⁻³ in p-type GaAs barrier layer 37 and p-type GaAs opticalconfinement layer 38 which overlie undoped quantum dots 36, and thesetwo layers therefore constitute p-type impurity regions.

In this construction, holes are generated with a surface density of1×10¹¹ cm⁻² in p-type GaAs barrier layer 37 and p-type GaAs opticalconfinement layer 38 and flow into quantum dots 36. At this time, twoholes are injected into each quantum dot, and these holes consequentlyfill the ground level of the valence band of each quantum dot.

As a result, in the quantum dot laser of the second example as well, theelectron density of the ground level of the conduction band of quantumdots drops during laser oscillation, and the process of relaxation ofconduction electrons from an excited level to the ground level of theconduction band is accelerated.

In the quantum dot laser of the second example, the relaxation time ofconduction electrons from an excited level to the ground level of theconduction band of quantum dots can be accelerated up to the order of 10ps, and high-speed modulation of 10 GHz or more can therefore berealized.

Although p-type impurity (Be) was directly doped into InAs quantum dots26 in the first example, it was found that this type of construction wasproblematic due to the occurrence of flaws in the quantum dots and theconsequent drop in emission efficiency, as described hereinabove. In thesecond example, rather than doping the p-type impurity (Be) into thequantum dots, the p-type impurity is doped in semiconductor layers 37and is 38 formed in the vicinity of quantum dots 36. Holes are thengenerated at a surface density of 1×10¹¹ cm⁻² in these two semiconductorlayers 37 and 38, and these holes are caused to flow into quantum dots36.

Thus, the second example not only obtains the same effects as the firstexample but also allows an improvement in modulation speed whilesuppressing decrease in the light emission efficiency of the quantumdots.

Third Example

FIG. 10 shows the construction of the third example of a quantum dotlaser in which the semiconductor quantum dot device of the presentinvention is applied.

The quantum dot laser of the third example is a construction in whichp-type InAs wetting layer 45 is formed in place of undoped InAs wettinglayer 25 which was shown in the first example, and undoped InAs quantumdots 36 which were shown in the second example are formed in place ofp-type InAs quantum dots 26 which were shown in the first example. Theconstruction is otherwise identical to the semiconductor laser of thefirst example, and constituent elements which are the same as elementsin the first example are therefore identified by the same referencenumerals and redundant detailed explanation is here omitted.

The quantum dot laser of the third example was fabricated by thefollowing procedure using an MBE apparatus as in the first example andsecond example.

First, as in the first example, n-type AlGaAs cladding layer 22, undopedGaAs optical confinement layer 23, and undoped GaAs barrier layer 24 arefirst successively grown on n-type GaAs substrate 21.

InAs is then grown on undoped GaAs barrier layer 24 while supplyingberyllium to form p-type InAs wetting layer 45 (thickness of 0.3 nm). Aplurality of undoped InAs quantum dots 36 are next formed on this p-typeInAs wetting layer 45 as in the second example, and undoped GaAs barrierlayer 27 and undoped GaAs optical confinement layer 28 are successivelygrown on these undoped InAs quantum dots 36.

Finally, as in the first example, p-type AlGaAs cladding layer 29 andp-type AlGaAs cap layer 30 were successively formed on undoped GaAsoptical confinement layer 28 to obtain the semiconductor laser shown inFIG. 10.

As shown in FIG. 10, undoped InAs quantum dots 36 of the third exampleare covered by undoped GaAs barrier layer 27. In addition, quantum dotstructure 70 b is formed by p-type InAs quantum well layer (wettinglayer) 45 and the plurality of undoped InAs quantum dots 36 formed onthis p-type InAs quantum well layer 45. Furthermore, quantum dotstructure 70 b and undoped GaAs barrier layers 24 and 27 formed aboveand below this quantum dot structure 70 b constitute active layer 71 bof the quantum dot laser of the present example.

If the energy band structure is considered in the semiconductor laser ofthe third example, the energy level of p-type InAs wetting layer(quantum well layer) 45 is lower than the energy levels of undoped GaAsbarrier layer 27 and undoped GaAs optical confinement layer 28. Inaddition, the energy level of undoped InAs quantum dots 36 is lower thanthe energy level of p-type InAs wetting layer 45. The nature ofconduction electrons is to flow from a region of high energy level to aregion of low energy level, and the conduction electrons therefore flowfrom GaAs layers 27 and 28 to wetting layer 45, and from wetting layer45 to quantum dots 36.

On the other hand, potential barriers against holes, these potentialbarriers being referred to as “spikes” or “notches,” occur as a resultof the discontinuity of the energy band at the hetero-junction interfaceof GaAs barrier layer 27 and InAs wetting layer 45. These potentialbarriers obstruct the movement of holes from GaAs barrier layer 27 toInAs wetting layer 45.

InAs wetting layer 45 is therefore preferable to GaAs barrier layer 27as the p-type impurity region for injecting holes into InAs quantum dots36 due to the more efficient injection of holes into InAs quantum dots36.

In the quantum dot laser of the third example, p-type impurity is dopedin InAs wetting layer 45, and GaAs barrier layer 27 is undoped. As aresult, not only can the same effects as the first example be realized,but holes can also be efficiently introduced into InAs quantum dots 36from p-type InAs wetting layer 45 without any obstruction of thetransition of conduction electrons.

Fourth Example

FIG. 11 shows the construction of the fourth example of the quantum dotlaser in which the semiconductor quantum dot device of the presentinvention is applied.

The fourth example of the quantum dot laser is a construction in whichundoped InAs quantum dots 36 which were shown in the second example areformed in place of p-type InAs quantum dots 26 which were shown in thefirst example, and in which p-type GaAs barrier layer 51 and undopedAlGaAs buried layer 52 are formed in place of undoped GaAs barrier layer27 which was shown in the first example. The construction is otherwisethe same as that of the semiconductor laser of the first example, andconstituent elements which are identical to those of the first exampleare therefore identified by the same reference numerals and redundantdetailed explanation is here omitted.

The semiconductor quantum dot laser of the present example wasfabricated by the following procedure which uses an MBE apparatus.

First, as in the first example, n-type AlGaAs cladding layer 22, undopedGaAs optical confinement layer 23, undoped GaAs barrier layer 24, andundoped InAs wetting layer 25 were successively grown on n-type GaAssubstrate 21.

Next, as in the second example, a plurality of undoped InAs quantum dots36 were formed on undoped InAs wetting layer 25, and further, undopedAlGaAs buried layer 52 (thickness of 8 nm) was selectively formed onundoped InAs wetting layer 25 so as to bury the gaps between the quantumdots. At this time, the thickness of undoped AlGaAs buried layer 52 wasequal to the height of quantum dots (8 nm).

Continuing, p-type GaAs barrier layer 51 (thickness of 20 nm, andcarrier concentration of 5×10¹⁶ cm⁻³) was grown on the quantum dots andundoped AlGaAs buried layer 52, following which undoped GaAs opticalconfinement layer 28, p-type AlGaAs cladding layer 29, and p-type AlGaAscap layer 30 were successively grown on this p-type GaAs barrier layer51 as in the first example, whereby the semiconductor laser shown inFIG. 11 was obtained.

As shown in FIG. 11, undoped InAs quantum dots 36 of the fourth exampleare entirely covered by undoped AlGaAs buried layer 52 with theexception of the upper surfaces of undoped InAs quantum dots 36, and theupper surfaces of undoped InAs quantum dots 36 are covered by p-typeGaAs barrier layer 51. Quantum dot structure 70 a is formed by undopedInAs quantum well layer (wetting layer) 25 and the plurality of undopedInAs quantum dots 36 formed on this undoped InAs quantum well layer 25.Furthermore, quantum dot structure 70 a together with underlying undopedGaAs barrier layer 24 and overlying p-type GaAs barrier layer 51, andundoped AlGaAs buried layer 52 constitute active layer 71 c of thequantum dot laser of the present example.

In the quantum dot laser of the fourth example, AlGaAs buried layer 52,which has an energy level which is higher than GaAs, is buried at theside surfaces of undoped InAs quantum dots 36 as described in theforegoing explanation, and p-type GaAs barrier layer 51 is formed incontact with the upper surfaces of quantum dots 36.As a result, only theupper surfaces of InAs quantum dots 36 contact p-type GaAs barrier layer51.

The flow path of carriers to undoped quantum dots 36 of quantum dotlaser of the fourth example is next considered.

Carriers flow into undoped quantum dots 36 from below quantum dots 36 byway of undoped InAs wetting layer 25. The formation of undoped AlGaAsburied layer 52 at the side surfaces of quantum dots 36 prevents theflow of carriers into quantum dots 36 from these side surfaces. On theother hand, carrier flows in from above quantum dots 36 from p-type GaAsbarrier layer 51 (which has a smaller band gap than AlGaAs buried layer52) which contacts the upper surfaces of quantum dots 36. As a result,holes are introduced into quantum dots 36 directly from p-type GaAsbarrier layer 51 and enter the ground level of the valence band.

In the semiconductor laser of the fourth example, conduction electronsinjected from the outside enter undoped quantum dots 36 by way ofundoped InAs wetting layer 25 during operation (during laseroscillation) and consequently undergo radiative recombination insidequantum dots 36 with holes which have been injected from outside thesemiconductor laser. The conduction electrons injected from the outsidedo not recombine inside wetting layer 25 with holes which have beenintroduced in advance.

As a result, the semiconductor quantum dot laser of the present examplenot only provides the same effects as the first example but can alsoraise the efficiency of radiative recombination in quantum dots 36.

Fifth Example

FIG. 12 shows the construction of an optical amplifier (quantum dotoptical amplifier) in which the semiconductor quantum dot device of thepresent invention is applied.

The semiconductor optical amplifier of the present example is aconstruction in which a semiconductor layer is formed which has alaminated structure identical to the quantum dot laser of the secondexample shown in FIG. 9, and in which low-reflection films 61 a and 61 bare applied to and formed at both end surfaces of this semiconductorlayer. The construction of the semiconductor layer is equivalent to thatof the second example and redundant explanation is therefore hereomitted.

As shown in FIG. 12, in the semiconductor optical amplifier of thepresent example, signal light introduced into the semiconductor opticalamplifier by low-reflection films 61 a and 61 b which are formed at bothend surfaces of the laminated structure (optical waveguide) is amplifiedby stimulated emission in active layer 71 a having quantum dot structure70 a.

Gain in the ground level of quantum dots 36 which is lost by stimulatedemission is recovered by the relaxation of conduction electrons andholes to the ground levels of the conduction band and valence band ofquantum dots 36. Thus, as in the quantum dot laser of the secondexample, the injection of holes in advance in the ground level of thevalence band of quantum dots 36 enables an acceleration of the processof relaxation of conduction electrons. As a result, opticalamplification can be carried out that follows signal light whichundergoes high-speed modulation in excess of 10 GHz.

[Modifications]

Although examples have been described in the above-described first tofourth examples in which the semiconductor quantum dot device of thepresent invention was applied to a semiconductor laser, the presentinvention is not limited to this form and can be applied to any otheroptical device that uses quantum dots.

Further, although an example was described in which the semiconductorquantum dot device of the present invention was applied to asemiconductor optical amplifier in the fifth example, the presentinvention is not limited to this form and can be applied in otheroptical devices such as semiconductor optical switches for implementingsignal light switching and semiconductor wavelength conversion elementsfor converting the wavelength of signal light. Accordingly, the presentinvention can be applied to high-speed switching and wavelengthconversion of signal light by the same construction and principles asthe semiconductor optical amplifier.

Further, as devices that use quantum dots, applications are alsopossible to field effect transistors, single-electron control memories,or optical memories.

Although examples were described in the above-described first to fifthexamples in which InAs was used for the wetting layer and quantum dots,the present invention is not limited to this form, and the wetting layerand quantum dots may employ semiconductors other than InAs. For example,InGaAs, GaInNAs, InGaP, and InGaN may also be used as the material thatmakes up the wetting layer and quantum dots.

1. A semiconductor quantum dot device provided with a plurality ofquantum dots, comprising: a p-type impurity region disposed adjacent tosaid quantum dots, a p-type impurity being injected into said p-typeimpurity region for generating holes for filling a ground level of avalence band of said quantum dots.
 2. The quantum dot device accordingto claim 1, comprising: a p-type semiconductor cladding layer and ann-type semiconductor cladding layer which are arranged with said quantumdot interposed; wherein said p-type impurity region is formed betweensaid quantum dots and said p-type semiconductor cladding layer.
 3. Thequantum dot device according to claim 1, wherein said p-type impurityregion is a semiconductor wetting layer doped with p-type impuritieswhich adjoins said quantum dots.
 4. The quantum dot device according toclaim 1, wherein said p-type impurity region is formed on an uppersurface of said quantum dots whose side is surrounded with an undopedsemiconductor buried layer.
 5. The quantum dot device according to claim4, comprising: a semiconductor buried layer which buries gaps betweensaid quantum dots; wherein a band gap of said p-type impurity region issmaller than a band gap of said semiconductor buried layer.
 6. Thesemiconductor quantum dot device according to claim 1, wherein theconcentration of said p-type impurity is a value for introducing, intosaid quantum dots, holes of a number which is equal to or greater than anumber of states which can exist in the ground level of the valence bandof said quantum dots.